Broadly reactive antitumor cytotoxic cell activity can be induced in vitro in normal mouse spleen cells by polyinosinic acid, supernatants of Con A-stimulated cells or fetal calf serum. Like primary in vitro responses to allogeneic transplantation antigens, activity against syngeneic tumor cells normally peaks only after 4-5 days of culture. Syngeneic irradiated sarcoma cells alone only modulate and will not induce such activity. The effector cells appear to recognize a common determinant on widely diverse target cells or else express multiple receptors of different specificities at the single cell level. The effector cells do not react with normal lymphoid cells. Freshly dispersed tumor cells from primary MCA-induced sarcomas are also susceptible to these syngeneic effector cells, and the tumor cells are not injured by natural killer cells from syngeneic nude mouse spleens. Spleen cells from individual mice from crosses involving high and low responder strains have been tested. Analysis of parents, F1's and backcross mice indicates that responsiveness is under the control of multiple dominant or codominant genes.